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Ocean acidification - the rise in ocean acidity due to the increased absorption of carbon dioxide (CO₂) - is often thought of as consequence of climate change. However, it is actually a separate, albeit very closely-related problem.

Ocean acidification is often referred to as "the other CO₂ problem" because, like climate change, it is primarily a result of the increased emissions of this gas. Despite their common driver, though, the processes and impacts of ocean acidification and climate change are distinct. It should not be assumed that policies intended to deal with the climate will simultaneously benefit the oceans.

The current emphasis of global climate policies on a warming target is a case in point.

A narrow focus on temperature stabilisation, for example, opens the door for policy interventions that prioritise the reduction of greenhouse gases other than carbon dioxide. This is because non-CO₂ greenhouse gases - like methane and nitrous oxide, which can arise from agricultural and industrial processes - typically have a higher global warming potential and might even be less costly than CO₂ to reduce.

In addition, several geoengineering schemes have been proposed to reduce the impacts of a warming climate. Yet such schemes often do nothing to address emissions, and may even exacerbate carbon absorption in the oceans.

Reducing CO₂ - the only long-term solution

The most important step in addressing both climate change and ocean acidification, and ultimately the only way to avoid the most serious impacts of both, is the reduction of carbon dioxide emissions.

Long-term policy targets designed to guide emission reductions to a level that would avoid unacceptable consequences should consider both ocean acidification and climate change. Interestingly, it is in doing this that we see the solutions to these two global issues converge.

Countries have largely agreed that there is a desire to limit global temperature increases to no more than 2℃ above pre-industrial temperatures. This is a desire that requires us to drastically reduce our carbon dioxide emissions. Indeed, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report found that for a 66% chance of remaining below 2℃ we can emit less than 1,010 billion tonnes of carbon dioxide, or about one-third of our carbon budget.

In fact, such a target is in line with the most ambitious atmospheric carbon concentration scenario (called RCP2.6) used by the IPCC to model climate impacts.

A recent study in the journal Science conducted by J.P. Gattuso and colleagues modelled this same IPCC scenario and found that exceeding it would have wide-ranging consequences for marine life, marine ecosystems, and the goods and services they supply to humanity. However, as with climate change, many of the worst impacts of rising acidity could be avoided by following or remaining below this trajectory.

The most critical feature of this scenario with regards to ocean acidification is a reduction of carbon dioxide to net zero emissions by no later than 2070.

But, as Gattuso's team importantly note, even achieving zero emissions within this timeframe would not prevent substantial ocean acidification. Coral reefs and shellfish populations will remain especially vulnerable.

This is true for climate change impacts as well. And it is the reason that many, particularly those living in developing and low-lying island states, wish to see the long-term goal for global temperature rise reduced to 1.5℃.

In effect, this means that reducing net carbon dioxide emissions to zero must happen even sooner than 2070. Ocean acidification, therefore, provides the impetus for additional urgency in agreeing to stringent timeframes for reducing CO₂ emissions.

Net zero emissions on the table at Paris?

We are fast approaching the next round of climate talks on the UN Framework Convention on Climate Change in Paris. If we are to see any meaningful global climate pact emerge, ocean acidification must sit firmly alongside climate change on the negotiation table.

Given the double threat that ocean acidification and climate change poses to some of the most vital goods and services underpinning human welfare, including food security, economic development, and the viability of ecosystems, it is crucial that world leaders set sharp emission reductions square in their sights.

Promisingly, up for negotiation in Paris is language that could see parties agreeing to net zero emissions. This would indeed be a very welcome, and ultimately necessary, development.

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